Frogeye leaf
spot of soybean has been detected in the Piedmont and the northeastern
counties of North Carolina. This potentially destructive disease of soybean
has not been common in North Carolina in the last 20 years, because the
majority of varieties grown were resistant. Frogeye leaf spot caused severe
soybean yield losses in the mid-South and Mississippi Delta regions in
1989. While it is still too early to assess the current importance of
this disease in North Carolina, many varieties released in the last 5-10
years are susceptible. Frogeye leaf spot, as the name indicates, is primarily
a foliar disease of soybean. The fungus, Cercospora sojina, which
causes this disease, however, can be seedborne. Frogeye leaf spot is most
likely to become a problem if infected seed is planted or if the disease
occurred in the previous year's soybean crop and the land is not rotated.
Extended periods of wet weather during the growing season will favor disease
development.

Lesions on
leaves are circular to angular spots which vary in size from less than
1 mm to 5 mm in diameter. Lesions are first visible on the upper surface
of the leaf. These lesions are distinctive in that the brown spots are
surrounded by a narrow red or dark reddish-brown margin. As lesions age,
the central area becomes ash-grey to light brown. Older lesions are light
to dark brown and frequently are translucent, having a grey to white center
which may contain minute dark spots. Smaller lesions may coalesce to form
larger, irregular spots on leaves. During a wet year, symptoms of Frogeye
leaf spot may appear uniformly over the foliage. In years with intermittent
wet periods, the symptoms may appear layered within the plant canopy since
only young leaflets are susceptible to infection. When plants are heavily
infected, leaves may die and fall prematurely. This can result in early
defoliation of soybean plants. If high rainfall and humidity persist,
stems and seeds also may become infected. Lesions on pods are reddish
brown, may appear sunken, and are circular or elongate in shape. Older
lesions on pods become brown to gray, usually with a narrow, dark-brown
border.

The most
important means for managing this disease in soybean is avoidance. Planting
resistant varieties or high quality seed of certified varieties free of
this pest are the best packages for management. If you detected or think
you detected frogeye leaf spot in your soybean crop last year, there are
several actions you should take. The first step would be selection of
a resistant variety. If you cannot plant a resistant
variety (SOY006), fields which had frogeye leaf spot last year should
not be planted to soybean for at least one year and preferably two. The
fungus which causes this disease will survive in crop residue. By all
means, avoid planting seed from fields which may have been infected with
frogeye leaf spot the previous year. The seed-borne aspect of this disease
is probably the most important form of spread over long distances. Soybean
varieties differ in their resistance to this pathogen. Some public varieties
and many varieties developed by private companies appear to be immune
while many other varieties possess resistance to certain races of the
pathogen. Since we don't know which races are prevalent in North Carolina,
selection of a variety with a high level of resistance or immunity is
preferable. If this disease was not present last year, it is not necessary
to select a variety solely on the basis of frogeye leaf spot resistance.
However, if the disease was detected in your crop last year a resistant
variety (SOY006) would be an appropriate choice.

When scouting
soybean fields for weeds and insects, take notes on the severity and prevalence
of foliar diseases. If you suspect frogeye leaf spot in your crop, contact
your County Extension Agent and have symptomatic plants sent to the Plant
Disease and Insect Clinic, North Carolina State University, at the first
opportunity. Should you encounter this disease or if you have fields that
were suspect last year, control methods are:

plant-resistant
varieties,

rotate
fields to corn or cotton (1-2 years),

treat
seed which may be infected with a fungicide labeled for seed treatment,

plow under
any crop residues on the soil surface, and

the use
of a foliar fungicide (Table 1) at the R2 and R5 soybean growth stage
may be beneficial if the disease is present and environmental conditions
that favor disease development occur. Fungicides, however, are not generally
recommended in North Carolina since we rarely encounter disease pressures
sufficient to reduce soybean yield below the cost of treatment.

Table
1. Foliar fungicides for control of frogeye leaf spot on soybean

Fungicide

Rates
& Remarks(*)

Benomyl

0.5
- 1.0 lb/acre

Chlorothalonil

2.0
- 3.5 pts/acre

(*)Make
two applications, one at R2-3 growth stage and a second application 14-21
days later. Do not graze treated fields or feed hay or vines to livestock.

Published
by North Carolina Cooperative Extension ServiceDistributed
in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment
and program opportunities are offered to all people regardless of race,
color, national origin, sex, age, or disability. North Carolina State
University at Raleigh, North Carolina A&T State University, U.S. Department
of Agriculture, and local governments cooperating.

Recommendations
of specific chemicals are based upon information on the manufacturer's
label and performance in a limited number of trials. Because environmental
conditions and methods of application by growers may vary widely, performance
of the chemical will not always conform to the safety and pest control
standards indicated by experimental data.

Recommendations
for the use of chemicals are included in this publication as a convenience
to the reader. The use of brand names and any mention or listing of commercial
products or services in this publication does not imply endorsement by
the North Carolina Cooperative Extension Service nor discrimination against
similar products or services not mentioned. Individuals who use chemicals
are responsible for ensuring that the intended use complies with current
regulations and conforms to the product label. Be sure to obtain current
information about usage and examine a current product label before applying
any chemical. For assistance, contact your county North Carolina Cooperative
Extension Service agent.

Last
update to information: May 2000
Last checked by author: May 2000
Web page last updated November 2000 by A.V.
Lemay.